Line a 9'' by 13'' baking sheet tray with parchment paper. In a small sauce pot, melt butter over medium heat. Turn off heat and add sugar. Stir to combine. Heat mixture over medium-high heat until butter-sugar solution reaches 300°F on a candy thermometer.
In the meantime, sprinkle ½ cup of chocolate chips and ½ cup of toasted, chopped hazelnuts onto prepared parchment paper.
Once butter-sugar solution has reached 300°F, remove from heat and let cool for 3 minutes. Stir in vanilla paste. Then pour over chocolate/hazelnut on the parchment lined sheet. Let cool for 5 minutes
Pour remaining cup of chocolate chips over parchment. Let chips sit for a couple minutes until slightly melted. Using an offset spatula, spread chocolate into an even layer. Sprinkle remaining chopped hazelnuts on top. Sprinkle maldon salt or fleur de sel as desired. Let sit at room temperature until chocolate sets. Once chocolate and toffee have cooled, cut to your desired shapes and sizes.